Principal Investigator / Author(s): Atkinson, Roger & Janet Arey

Contractor: UC Riverside

Contract Number: 97-312

Ozone pollution in urban areas is the result of the reactions of volatile organic compounds, including alkanes, with oxides of nitrogen under the influence of sunlight. Alkanes are significant components of gasoline and diesel fuels and of vehicle exhaust, and alkanes containing 9-13 carbon atoms are important constituents of mineral spirits. In the atmosphere, alkanes react with the hydroxyl (OH) radical to form a suite of products, certain of which are presently difficult to identify and quantify. Of particular interest because of their potential impact on ozone production is the formation of organic nitrates from the OH radical-initiated reactions of alkanes in the presence of NOx. Because NO2 which would otherwise be photolyzed to form ozone is sequestered in the organic nitrates, accurate measurements of the amounts of organic nitrates formed, including hydroxyalkyl nitrates, is critical to predictions of ozone formation from alkane photooxidations. The formation yields of alkyl nitrates from n-hexane, n-heptane and n-octane were measured to be 0.141 ± 0.020, 0.178 ± 0.024, and 0.226 ± 0.032, respectively. These alkyl nitrate yields are ~35% lower than previous data reported from this laboratory in the early 1980's. Through the use of negative ion atmospheric pressure chemical ionization mass spectrometry, hydroxyalkyl nitrate products were identified from the n-pentane, n-heptane and n-octane reactions for the first time and the presence of hydroxycarbonyl products was confirmed. The yields of the hydroxycarbonyl and hydroxynitrate products were quantified relative to internal standards of similar chemical structure. We then investigated the atmospheric chemistry of three representative linear, branched and cyclic C10 alkanes, n-decane, 3,4-diethylhexane and n-butylcyclohexane. Using a relative rate method, rate constants for reactions of the OH radical with n-decane, 3,4-diethylhexane and n-butylcyclohexane of (in units of 10 -12 cm 3 molecule -1 s -1 ) were measured to be 12.5 ± 0.4, 7.43 ± 0.48 and 15.8 ± 0.6, respectively, where the errors do not include uncertainties in the rate constant for the n-octane reference compound. The products identified and quantified were: 2-, 3-, 4- and 5-decyl nitrate, hydroxycarbonyls and hydroxynitrates from the n-decane reaction; acetaldehyde, propanal, 3-pentanone, 3-pentyl nitrate, hydroxycarbonyls and hydroxynitrates from the 3,4-diethylhexane reaction; and butanal, cyclohexanone, hydroxycarbonyls and hydroxynitrates from the n-butylcyclohexane reaction. These observed gas-phase products account for a significant fraction of the total products.